A so-called “lost-foam” casting process is a well-known technique for producing metal castings. A fugitive, pyrolizable, polymeric, foam pattern (including casting, gating, runners, and sprue) is covered with a thin (typically in the range of 0.25–0.5 mm), gas-permeable refractory coating/skin such as mica, silica, alumina, or alumina-silicate, for example. The pattern is embedded in compacted, unbonded sand to form a mold cavity within the sand. Molten metal is then introduced into the mold cavity to melt, pyrolyze, and displace the pattern with molten metal.
Gaseous and liquid decomposition/pyrolysis products escape through the gas-permeable, refractory skin and into the interstices between the unbonded sand particles. The casting rate or rate at which the molten metal enters the mold cavity is limited by the rate the advancing molten metal front can displace the pattern from the cavity. This is affected by the thickness and permeability of the refractory skin/coating. Typical fugitive polymeric foam patterns comprise expanded polystyrene foam (EPS) for aluminum castings and copolymers of polymethylmethacrylate (PMMA) and EPS for iron and steel castings, for example.
The polymeric foam pattern is made by injecting pre-expanded polymer beads into a pattern mold to impart the desired shape to the pattern. For example, raw EPS beads (typically 0.2 to 0.5 mm in diameter) containing a blowing/expanding agent (e.g. n-pentane) are: (1) first, pre-expanded at a temperature above the softening temperature of polystyrene and the boiling point of the blowing agent; and (2) molded into the desired configuration in a steam-heated pattern mold which further expands the beads to fill the pattern mold. Complex patterns and pattern assemblies are made by molding several individual mold segments, and then joining the mold segments by gluing, for example, to form the pattern or pattern assembly.
The molten metal may be either gravity-cast meaning poured from an overhead ladle or furnace, or countergravity-cast. In gravity-cast lost-foam processes, the metallostatic head of the molten metal in the sprue and pouring basin is the driving force for filling the mold cavity with molten metal. Countergravity-cast lost-foam processes involve causing the molten metal to flow upwardly by vacuum or low pressure into the mold cavity from an underlying vessel such as a furnace, for example.
Gravity-cast, lost-foam processes are known that top-fill the mold cavity by pouring the molten metal into a basin overlying the pattern so that the molten metal flows downwardly into the mold cavity through a gating system located above the pattern. Other gravity-cast methods bottom-fill the mold cavity by pouring the molten metal into a vertical sprue that lies adjacent the pattern. The sprue extends from above the mold cavity to below the mold cavity for filling the mold cavity through a gating system located beneath the pattern so that the molten metal flows vertically upwardly into the mold. Additionally, gravity-cast methods can side-fill the mold cavity by pouring the molten metal into a pattern that forms a vertical sprue which lies adjacent the mold cavity. The vertical sprue communicates with the mold cavity via a plurality of vertically aligned runners and gates which horizontally fill the mold cavity from the side. The vertical sprue may be flanked by two or more mold cavities for making multiple castings with a single pour.
Molten metal flow in a lost foam mold is related to the density of the foam pattern. Casting engineers are often challenged with a part configuration which does not lend itself to castability. Features such as long straight rails cause metal to flow through a mold quickly while causing other areas to back-fill. The back-fill areas can be subject to defects such as folds. Computer simulation programs have been used to attempt to adjust gate area and location in an attempt to optimize flow patterns.
It would be desirable to develop a method and apparatus for controlling dispersion of molten metal in a mold cavity for a lost foam casting process wherein the filling of regions of the mold cavity which do not lend themselves to castability is maximized, an amount of backfill and other casting defects are minimized, and a flow pattern of molten metal and material properties of the resulting casting are optimized.